As countermeasures against exhaust gas of a diesel engine (simply engine, hereinafter), there is such a well-known conventional technique that particulate matter (PM) and the like in exhaust gas are collected by providing an exhaust gas filter (diesel particulate filter) in an exhaust gas path of an engine, and exhaust gas is restrained from being discharged into atmosphere (see Patent Documents 1 and 2 for example). If PM collected by the exhaust gas filter exceeds a prescribed amount, flowing resistance in the exhaust gas filter is increased and engine output is lowered. Therefore, PM accumulated on the exhaust gas filter is removed by raising temperature of exhaust gas, thereby restoring (regenerating) PM collecting ability of the exhaust gas filter in some cases. When the exhaust gas filter is not sufficiently regenerated even if the temperature of the exhaust gas is increased, it is possible to facilitate the regeneration of the exhaust gas filter by supplying unburned fuel into the exhaust gas filter to burn PM. Such reset regeneration technique is also known.